There is a strong interest in providing multimedia services to the users of communications networks, particularly mobile communications networks. By “multimedia service” it is generally intended a service in which a plurality of media can be used for supporting and/or performing the communication, such as, for example, voice, text, images, video, access to data (e.g. through the Internet and/or an electronic mailbox), generic files. Among these services, the so-called “combinational services” are attracting great attention of the mobile telephony operators. For the purposes of the present description, by “combinational service” it is generally intended a service through which a terminal of a (not necessarily mobile) communications network can simultaneously open and use two connections, typically a circuit (circuit-switched or CS) connection and a packet (packet-switched, PS) connection.
For example, a description of a service enjoyable through mobile terminals is given in the White Paper “Video sharing—Enrich your voice call with video”, by Nokia Corporation, publicly available for download at the Internet site:                http://www.nokia.com/BaseProject/Sites/NOKIA_MAIN—18022/CDA/Categories/Phones/Technologies/VideoSharing/_Content/_Static_Files/video_sharing_a4—2510.pdf        
The real time video sharing service described in this White Paper allows the users, during a telephone conversation, to easily enrich their communication. One or the other of the users can share a live video taken by a video camera or video clips from the terminal. Both the users see the same video and can discuss about it while they are continuing their voice call.
In another White Paper, entitled “White Paper: IP Convergence Based On SIP: Enhanced Person-To-Person Communications”, publicly made available by Forum Nokia for download at the Internet site                http://www.forum.nokia.com/main/1,040,00.html?fsrParam=2-3-/main.html&fileID=5336the use of the SIP protocol is described for the establishment of connections between terminals capable of supporting the IP protocol. As described in the White Paper, in order to communicate, IP-based applications must have a mechanism to reach the correspondent. Today, fixed and mobile telephony networks perform this critical task of establishing a connection. By dialing the other user's telephone number, the network can establish an ad-hoc connection between any two terminals. This critical connectivity capability still does not exist widely in the Internet. According to this White Paper, SIP-based sessions management, complemented by other critical mobile networks capabilities (i.e., authentication, roaming, and network interconnection provided by the IMS standard) provides the required structure. With the implementation of such a system, it is possible to establish an IP connection between two terminals. Once the connection is established, it can be used to exchange all types of communication media (voice, video, content, etc.). Like HTTP, SIP is a text-based client-server protocol. SIP was designed to establish, modify and terminate multimedia sessions or calls, and it differs from the HTTP in the fact that a “SIP terminal” (or User Agent, UA) may act as both a client and a server. Therefore it is possible to establish a client-to-client communication.        
PCT patent application no. WO 2005/027460 discloses a method of establishing a combinational multimedia session between at least two end user terminals, the method comprising: discovering at each end user terminal the end user authorization to use the multimedia service; subsequently establishing a circuit switched connection between the end user terminals via one or more telecommunication networks; upon successful discovery of end user authorization to use the multimedia service and prior to or following the establishment of said circuit switched connection, discovering at each end user terminal the multimedia capabilities of the or each other terminal; and whilst the circuit switched connection is established, establishing an IP multimedia subsystem session between the end user terminals via one or more IP multimedia subsystem networks, and transferring IP multimedia information between the user terminals, said multimedia information relating to a service supported by both or all user terminals.
As also explained in the above mentioned PCT patent application, IP Multimedia Subsystem (IMS) is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP multimedia services over 3G mobile communication networks. IMS provides key features to enrich the end-user person-to-person communication experience through the integration and interaction of services. IMS allows new rich person-to-person (client-to-client) as well as person-to-content (client-to-server) communications over an IP-based network (not necessarily mobile, or only mobile). The IMS makes use of the SIP protocol to set up and control calls or sessions between user terminals (or user terminals and servers, such as web servers).